[skottie] 'Add' layer blend mode support

BM now exports an enum for 'add' blending [1]. This corresponds to
SkBlendmode::kPlus.


[1] 86874c1018

Change-Id: I3c9a52e4055563f50bb68f460eda27fed1c4a411
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208220
Auto-Submit: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>

modules/skottie/src/Skottie.cpp

@@ -212,6 +212,7 @@ static SkBlendMode GetBlendMode(const skjson::ObjectValue& jobject,
         SkBlendMode::kSaturation, // 13:'saturation'
         SkBlendMode::kColor,      // 14:'color'
         SkBlendMode::kLuminosity, // 15:'luminosity'
+        SkBlendMode::kPlus,       // 16:'add'
     };
 
     const auto bm_index = ParseDefault<size_t>(jobject["bm"], 0);